This invention generally relates to chemical processing apparatus, and more specifically, to a module for providing sufficient residence time to ensure that mixtures of reactants supplied to a chemical reactor have sufficient time to completely react with one another before the resulting product is output from the apparatus.
Methods of controlling and optimizing the production of chemical substances are well known. The control of parameters such as temperature, pressure, mixing conditions, relative volumes of the reactants, and the use of catalysts are generally well understood. Processing equipment for facilitating chemical reactions can be specifically designed for one particular reaction, or can be designed to facilitate the processing of more than just one specific chemical reaction.
Ideally, equipment designed for processing more than a single specific chemical reaction should be adjustable so that it can be used to facilitate a variety of different chemical reactions of interest. For example, because different chemical reactions require different temperature conditions, chemical processing equipment should enable temperature conditions in the equipment to be varied, so that the optimal temperature for a particular reaction can be provided.
While it is relatively easy to design processing equipment that enables temperature conditions to be adjusted, it is more challenging to design processing equipment that enables all relevant parameters to be adjusted. Reaction time (i.e., processing time) is another parameter that should be precisely controllable. Each different reaction is characterized by having an optimal processing time required to ensure that the reactants have sufficient time to completely react with one another. Ensuring that sufficient processing time is provided in a chemical processing system is critical to achieving good product yields and process efficiency. Providing too little time reduces product yield, because the product will include reactants that have not yet reacted to form the desired product. The processing time required in a given chemical processing system is a function of the specific reaction, the temperature of the reactants, and concentration and volume of the reactants. The amount of processing time provided by a given chemical processing system is a function of the volume of the reactor or reaction chamber within the system, and the flow rate of the reactants into the reactor (or reaction chamber). While the volume of the reaction chamber within a particular chemical processing system is generally fixed, flow rates can typically be varied throughout at least a limited range. Accordingly, the volume of the reaction chamber and the flow rates of the reactants should be selected to ensure that the reactants have sufficient time to thoroughly react.
However, there are instances in which additional processing time may be required for a particular reaction. In this case, it is well known in the art to provide xe2x80x9cresidence time chambersxe2x80x9d or modules down stream of a reaction chamber. Basically, a residence time chamber is a volume into which the already mixed reactants (exiting a reaction chamber or reactor) are diverted. The mixed reactants are held in the residence time chamber for a length of time sufficient to ensure that the desired reaction has been completed before the resulting product and any byproducts are directed into a collection vessel. Thus, residence time chambers can be beneficially incorporated into a chemical processing system when the volume of the reaction chamber and the available reactant flow rates within that chemical processing system cannot otherwise be readily modified to provide sufficient processing time.
Some residence time chambers are integral and unique to specific chemical processing systems and are not designed or intended to be replaceable or usable in other systems. U.S. Pat. No. 5,516,423 (Conoby) describes a chemical processing system in which a residence time is varied by controlling the effective volume of a reaction chamber supplied with a liquid effluent and a treatment chemical. A valve controls an outlet port from the reaction chamber, and a sensor determines whether a desired result has been achieved. Once the desired result has been achieved, the outlet valve is opened and the treated effluent is discharged. Accordingly, the outlet valve is controlled to vary the residence time of the reactants within the reaction chamber. However, new effluent continues to enter the reaction chamber during the residence time, so that the volume of effluent within the reaction chamber is not constant. To maximize product yield and quality, it is preferable to more precisely control the influx of reactants into a reaction chamber and the time of the reaction, rather than simply retaining the reactants in a reaction chamber until a desired result is achieved.
Many chemical processing systems are designed to operate with a constant volume of reactants in the reaction chambers, to ensure that unwanted byproducts caused by varying the relative concentrations of the reactants are not formed. For this reason, the method used to vary the residence time, as described in the above-noted patent, is generally not applicable to many types of reactions, or not desirable. It is preferable to include a separate residence time chamber down stream of the reaction chamber. Residence time chambers of many different types are known. Some residence time chambers are simply unobstructed chambers or fluid paths into which mixed reactants are directed, and the physical dimensions of the chamber or fluid path are such that the time necessary for the mixed reactants to traverse the chamber or fluid path provides the required residence time. Other residence time chambers include baffles or other types of flow restriction elements designed to increase the length of time required for the mixed reactants to traverse the chamber. Such residence time chambers are frequently integral components of chemical processing systems and are not designed to be selectively configurable to provide variable residence times. Furthermore, known residence time chambers typically incorporate a single fluid path, with a volume that controls the additional residence time provided by the residence time chamber.
A simple tube open at both ends can be used as a residence time chamber, and the additional reaction time provided by the device is strictly determined only by the flow rate of the reactants and the volume of the tube. A disadvantage of a simple tube type residence time chamber is that the additional reaction time can be only varied only by changing the flow rate (i.e., by changing the production rate of the chemical processing system, which will generally already have been optimized for a particular chemical production system), or by selecting and installing an appropriately sized tube from a stock of such tubes having different volumes.
It would therefore be desirable and preferable to provide a residence time chamber that is selectively configurable to provide variable residence times without changing to a different residence time chamber. Preferably, such a residence time chamber should incorporate a plurality of fluid paths, and the fluid paths should be individually controllable to provide the same or different residence times. It would further be desirable for such variable residence time chambers to be modular in nature, so that different residence time modules having different ranges of available residence times or other desired properties can be used in a chemical processing system or with a chemical reactor.
Recently, much attention has been directed to the use of microreactors for both research and development work, and the production of chemical products. Microreactors offer several clear advantages over more conventional macro-scale chemical processing systems. Accordingly, it would be desirable to provide a variable residence time chamber adapted to be used with microreactors. The prior art does not teach or suggest such a modular, variable residence time chamber.
In accord with the present invention, a variable residence time module is defined. Unlike the prior art, the present invention employs a single residence time module that can provide different duration residence times in response to control signals supplied by a control unit.
A variable residence time chamber in accord with the present invention includes a housing having an inlet port and an outlet port, an inlet valve, an outlet valve, and a plurality of residence time chambers disposed within the housing. The inlet valve and the outlet valve, respectively selectively couple the inlet port to one of the plurality of residence time chambers, and the outlet to one of the plurality of residence time chambers. A sufficient residence time for a reaction is provided by retaining the mixed reactants within the selected one of the plurality of residence time chambers for a time sufficient for the mixed reactants to completely react to form the desired product (or mixture of products). In the present invention, the reactants are generally not in motion except when the selected residence time chamber is being filled or emptied.
Preferably, in at least one embodiment, the housing includes a heat transfer media inlet, and a heat transfer media outlet. An inner volume of the housing is adapted to be substantially filled with a heat transfer media through the heat transfer media inlet, and spent heat transfer media exits through the heat transfer media outlet. Also, in at least one embodiment, the housing is fabricated from a material that provides a thermally insulating barrier between the ambient environment and the interior volume of the housing.
The configuration of the residence time chambers is not critical, and any of them can comprise either a substantially elongate tube, a coiled tube, or an irregular-shaped tube. Preferably, the plurality of residence time chambers are fabricated from a substantially chemically inert material. Examples of a suitable chemical inert material include metals, stainless steel, glass, plastics, and polymer coated materials.
One embodiment includes an additional inlet valve and an additional outlet valve, each selectively coupled with a selected different one of the plurality of residence time chambers. The inlet valves and outlet valves are preferably rotary valves that couple a selected residence time chamber to the inlet port, in the case of the inlet valves, and couple a selected residence time chamber to the outlet port, in the case of the outlet valves. It is anticipated that it will be useful to include markings on the inlet and outlet valves to enable an operator to determine which of the plurality of residence time chambers the inlet valve and the outlet valve are currently servicing.
In at least one embodiment, a residence time chamber support structure is included for supporting and mounting the plurality of residence time chambers within the housing. This residence time chamber support structure, and the associated plurality of residence time chambers are readily replaceable, so that a first residence time chamber support structure and a first plurality of residence time chambers can be replaced with a second residence time chamber support structure and a second plurality of residence time chambers, which may have different internal volumes.
Preferably, a prime mover is drivingly coupled to the inlet valve and the outlet valve, to selectively position the inlet valve and outlet valve automatically in response to a control signal. A power bus and a data bus are preferably included, the data bus being coupled to the prime mover to enable the residence time module to be automatically controlled by a separate control module, and the power bus electrically coupling the residence time module to a power source that provides an electrical current to energize the residence time module.
In yet another embodiment, at least one of the plurality of residence time chambers possess a diameter that is sufficiently small so as to enable a laminar flow within the residence time chamber. In still another embodiment, at least one of the plurality of residence time chambers include means that facilitate production of the desired chemical product. The means includes at least one of a glass bead packing, a silica packing, and a catalyst packing. Also included is a sensor that produces a signal indicative of a parameter relating to a reaction of the reactants within a residence time chamber, the sensor being either a temperature sensor, an electrochemical sensor, an optical sensor, or a pressure sensor.
Finally, in another embodiment, the variable residence time module includes at least one of a source of electromagnetic radiation, a source of sonic radiation, a source of alpha radiation, a source of beta radiation, a source of gamma radiation, a source of X-rays, a source of radioactive particles, a source of charged particles, a source of light, an electric field source, and a magnetic field source.
Another aspect of the present invention is directed to a method for selectively varying a residence time. The steps of this method are generally consistent with the functions of the elements of the residence time apparatus discussed above.